There are a variety of fiber optic sensors, such as Bragg grating sensors, fiber laser sensors, and interferometric sensors having the potential for measuring small changes in temperature, pressure or strain on or established in an optical fiber. Strains can be induced by physical, chemical or biological parameters, or by electromagnetic fields, and these sensors can be configured to measure accurately a variety of different parameters (measurands). Hence, it is known that optical fibers may be provided with claddings or coatings which react to particular measurands to establish strain within a fiber, this strain changing a detectable optical property of the fiber such that a particular parameter can be measured.
Such sensors are used in medical applications, and in various other applications including engineering and oil and gas exploration.
In relation to such sensors, the varying optical properties of the fiber at one or more sensing locations thereof can be provided by various known means. For example, sensing regions of the fiber may be configured to provide a form of “Fabry-Perot” (F-P) interferometer, whose resonance wavelength when interrogated by a suitable laser light source depends on strain established within the fiber. In such a system there are effectively spaced “mirrors” written into the fiber whose spacing determines the output wavelength which therefore changes with longitudinal strain within the fiber.
Alternatively, the sensing regions can be based on active or passive fiber Bragg gratings (FBG's) written into the optical fiber core. These gratings are made by producing periodic variations in the refractive index along a short section or sections of the core, and methods for fabricating such gratings are generally known in the art.